Fluorescent dye-penetrant inspection has been an established nondestructive testing method for many years. Traditionally, a component is prepared for inspection by exposing the surface to a low-viscosity fluid that acts as a carrier for a tracer material. As a result of interfacial tension and adsorption, the liquid penetrates into even the smallest surface-breaking cracks and voids. When exposed to ultraviolet light (300 nm to 400 nm wavelength) the tracer material fluoresces at a visible wavelength, thus revealing the presence of indications such as surface-breaking cracks and voids. The most common method for evaluating these indications is through visual observation.
A significant benefit of liquid-penetrant testing is its sensitivity to very small cracks. Surface-breaking cracks as small as a few microns in length can be detected using this nondestructive testing method. Efforts have been undertaken to extend the process of fluorescent dye-penetrant testing to the internal surfaces of critical components such as nuclear steam-generator tubes. These devices use fiber-optic imaging probes and endoscopes, as described in Olympus Corporation, U.S. Pat. No. 5,115,136 and Commissariat a I""Energie Atomique, U.S. Pat. No. 4,791,293, with a right-angle mirror, which xe2x80x9cpipexe2x80x9d the two-dimensional image from the tube surface to a remote viewing station using an optical fiber or fiber bundle. In order to rotate the observation assembly, the fiber-optic bundle must include a complex and signal-degrading optical slipring.
Commercial forward-viewing video probes that employ a miniature CCD camera (Welch Allyn, Inc., U.S. Pat. No. 5,202,758) also have been adapted for remote inspection of tubes and pipes that have been treated with dye-penetrant, again using a right-angle mirror to view the test-part surface. Because the two-dimensional image of the cylindrical part surface is obtained using a right-angle mirror, the image is distorted and difficult to interpret visually. These devices do not employ a slipring because of the combination of optical fibers (for light transmission to the tube) and electrical wires (for transmission of the electrical signals from the CCD camera to an external viewing monitor). Inspection is typically a slow process that requires an operator to manipulate the probe manually to various regions of the tube. The two-dimensional image typically is stored to video tape for visual evaluation or, in some cases, image-processing. However, efforts to automate the process through complex image-processing methods have largely proven excessively expensive and unreliable.
Although conventional fluorescent dye-penetrant testing is well accepted and has been in use for years, it is labor-intensive, time-consuming and difficult to convert the complex visual images into quantitative data in a cost-effective manner. Another method for remote nondestructive testing of tubular components using dye-penetrant has been developed by a Sweden-based organization (iP-TEC). As described in U.S. Pat. No. 5,554,800 the system provides a means for treating a surface deep within a steam-generator (or other tube) with photoluminescent dye-penetrant liquid. A plastic plug is then placed in physical contact with the area of concern and any penetrant that has wicked into a crack will be absorbed into the plug, thus creating a precise physical replica of the feature. The plug must then be removed manually from the delivery probe and exposed to UV light in a special viewing device. Features then can be measured manually, photographed and archived. A significant drawback associated with this system is the requirement for manual replacement of the replica plug. This is a significant problem when operating in radiation environments found in nuclear generating stations because it is time-consuming, labor-intensive and exposes the operators to radiation.
An important aspect of this invention is its use of a non-rotating illumination source and a non-rotating photodiode assembly, cooperating with a rotating passive optical scanning mirror assembly, thus eliminating the need for complex, expensive and signal-degrading sliprings. The ultraviolet illumination source and photodiode are mounted on-axis and do not rotate. De-coupling these two elements from the rotating components of the scanning probe eliminates the need to use electrical or optical sliprings to xe2x80x9cpipexe2x80x9d a complex two-dimensional visual image out of the probe via optical fibers. These fundamental differences provide substantial improvement and advantage over conventional video- and fiber-optic- based viewers, including dye-penetrant scanning systems.
The preferred scanning system described herein provides an automated means of rapidly and accurately inspecting a tube, pipe, or other cylindrical or enclosed cavity that have been treated with a photoluminescent penetrant medium. The invention includes: 1) a means of delivering ultraviolet radiation to the test-part surface; 2) a passive rotary scanning means that includes a plurality of lenses, reflectors and an optical filter; 3) an offset rotary drive means that is capable of causing the scanning means to rotate at up to several thousand revolutions per minute; 4) a solid-state photodiode that receives fluorescent light and transmits electrical signals that correspond to the presence of photoluminescent penetrant; and 5) an instrumentation station that provides a means to post-process the signals and a means of evaluating and displaying the condition of the part being inspected.
The passive optical scanning means is caused to rotate by the offset rotary drive means. Ultraviolet radiation from the illumination source is projected on-axis into the optical scanning means. A plurality of lenses focuses the radiation, and a reflecting prism directs the radiation onto the part surface. The result is a concentration of the ultraviolet radiation energy into a very small area (approximately 0.25 mm2). If the ultraviolet radiation strikes dye-penetrant that has been adsorbed into a crack or void, the penetrant will be caused to fluoresce at a visible wavelength.
The receiving lenses, also contained in the passive optical scanning means, capture the fluorescent radiation emanating from the test-part surface and project it through an optical filter and onto the photodiode. A series of electrical pulses corresponding to the presence of surface flaws is generated from the photodiode. The electrical signals are then ported out of the probe to signal-processing instrumentation, via electrical wires, and digitized for further processing. The optical scanning assembly is rotated and translated along the axis of the tube or pipe that is being inspected, creating a helical map of the part surface. By encoding the linear and angular position of the scanning means, an accurate and quantitative map of the digitized flaw pattern can be constructed.